I. Field of the Invention
The present invention relates to a method and apparatus for controllably depositing a thin film of material on a substrate and to the thin film products.
II. Background Information
Systems are known for depositing thin films of material on a substrate. One such system comprises a housing in which a high vacuum region may be formed; a crucible containing the material to be deposited; a heater for vaporizing that material in the crucible; a nozzle for ejecting the vaporized material from the crucible through the nozzle into the high vacuum region to form non-ionized atomic clusters of the material by resultant adiabatic expansion which are held together by van der Waals forces and which drift outward from the nozzle on an approximate line of sight basis; an electrode system for converting a percentage of the non-ionized clusters into ionized clusters; and an accelerator for accelerating the ionized clusters toward the target upon which the thin film is to be deposited.
Such a system, as is for example disclosed in U.S. Pat. No. 4,217,855 issued to Takaqi, provides for control of deposition rate primarily through control of vapor pressure within the crucible. In addition, the ion acceleration voltage is critical in obtaining the maximum deposition rate. Crucible temperature, the shape of the ejection nozzle, the deposition material employed, and the substrate material employed are also important in determining deposition rate.
Prior art systems of the Takaqi type recognize the possibility of reversing the polarity on the accelerator to impede the progress of the ionized clusters without affecting the progress of the non-ionized clusters in their movement toward the target. At least one system, that disclosed in U.S. Pat. No. 4,082,636 issued to Takaqi, contemplates an acceleration voltage which causes the ionized clusters to actually be repelled and thereby prohibits deposition of the ionized clusters on the target, at least during a portion of the operating time of the system.
An object of the present invention is to provide an alternative and improved method for controlling the rate of deposition of material forming a thin film on a target.
One prior art system disclosed in U.S. Pat. No. 4,197,814 issued to Takeqi et al. contemplates the deposition of two or more materials on a target. This system discloses the employment of two crucibles each having a separate heating mechanism for vaporizing a different material in each crucible. The nozzles on these two crucibles are disclosed as being directed toward a common target, thereby permitting both ionized and non-ionized atomic clusters from each crucible to be simultaneously deposited on that target. With such a system, the temperatures of the crucibles may be independently regulated so as to regulate the pressures of the vapors contained in those crucibles to thereby independently control the deposition rates of the two materials. However, any such change in the crucible temperature takes a considerable amount of time in comparison to the total deposition time typically required and thereby renders instantaneous control of the deposition rates effectively impossible.
It is also known that systems of the type referred to above are capable of depositing an oxide of a material in a crucible, through the introduction of oxygen gas into the high vacuum region adjacent the target. However, it is difficult to control the rate and degree of oxidation using this methodology.
In operating these prior art systems a layer of the film of a particular composition is deposited, the deposition process is stopped, the crucible temperature or pressure within the high vacuum region is altered, and thereafter the deposition process begun again to deposit a new composition. As a consequence, rather than being smoothly varying, a material having a series of discrete, discontinuous step changes in composition is obtained.
Optical filters or antireflection coatings typically include these discrete stacked layers of dielectric material, each layer having different optical properties. New applications for these filters indicate the need to develop systems having the capacity to deposit thin film layers which have substantially smoothly or continuously varying material properties as a function of film thickness, rather than a series of discrete step-wise changes. The prior art systems are incapable of providing smoothly varying material properties because the vaporization rate (and consequently the deposition rate) responds to changes in crucible temperature too slowly.
It is also an object of the present invention to provide a system which permits a thin layer to be deposited which has smoothly or continuously varying material properties as a function of thickness of that layer, such as a film that has a continuously graded index of refraction caused by the continuously changing composition of the layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from that description or may be learned by practice of the invention.